Fix failure for empty attribute match

Running the following invalid attribute search, properly throws an
IllegalArgumentException.

    scala> <x/> \ "@"
    java.lang.IllegalArgumentException: @

There's no such thing as an empty attribute.

However, when the improper matching value is used against more than
just one element, no error is thrown, just an empty NodeSeq is
returned:

    scala> <x><y/><z/></x>.child \ "@"
    res1: scala.xml.NodeSeq = NodeSeq()

It should be a failure.

This was identified while writing ScalaCheck property tests.

Author: ashawley

shared/src/main/scala/scala/xml/NodeSeq.scala

@@ -117,6 +117,7 @@ abstract class NodeSeq extends AbstractSeq[Node] with immutable.Seq[Node] with S
     that match {
       case ""                                        => fail
       case "_"                                       => makeSeq(!_.isAtom)
+      case "@"                                       => fail
       case _ if (that(0) == '@' && this.length == 1) => atResult
       case _                                         => makeSeq(_.label == that)
     }

shared/src/test/scala/scala/xml/AttributeTest.scala

@@ -154,4 +154,13 @@ class AttributeTest {
     assertEquals(List(Group(Seq(Text("1"))), Group(Seq(Text("2")))), barList)
   }
 
+  @Test(expected=classOf[IllegalArgumentException])
+  def invalidAttributeFailForOne: Unit = {
+    <x/> \ "@"
+  }
+
+  @Test(expected=classOf[IllegalArgumentException])
+  def invalidAttributeFailForMany: Unit = {
+    <x><y/><z/></x>.child \ "@"
+  }
 }